1. Field of the Invention
The present invention relates to a method of driving a field emission cold cathode device having a minute emitter and minute gate electrodes disposed near the emitters, and a field emission cold cathode electron gun.
2. Description of the Related Art
Field emission cold cathode devices have a sharply pointed conical emitter, a gate electrode having an opening of submicron size and disposed near the emitter, and an anode electrode. When voltages are applied to the gate electrode and the anode electrode, a high electric field is concentrated on the tip end of the emitter to enable the emitter to emit electrons from its tip end in a vacuum.
As shown in FIG. 1 of the accompanying drawings, an emitter 1 is mounted on a substrate 2 which is either electrically conductive or electrically insulative with an electrically conductive film. A gate electrode 3 is connected to a gate power supply 14 and disposed a in surrounding relation to the tip end of the emitter 1. An anode electrode 5 is connected to an anode power supply 6 and disposed in a confronting relation to the tip end of the emitter 1.
Positive voltages with reference to OV are applied from the gate electrode power supply 14 and the anode electrode power supply 6 respectively through the gate electrode 3 and the anode electrode 5 to the emitter 1 for enabling the emitter 1 to emit electrons from its tip end.
The electrons are normally emitted from a small region of the tip end of the emitter which has a length of 10 nm or less. The emitted electrons are affected by gases remaining in the vacuum.
When the gate voltage applied to the gate electrode is controlled as shown in FIG. 2a of the accompanying drawings, an emission current which represents electrons emitted from the emitter flows according to the Fowler-Nordheim equation: ##EQU1## where J is the work function at the tip end of emitter and V is the field intensity generated by the configuration of the emitter near its tip end, and .alpha., .beta. are constants, .alpha. depending on the area from which the electrons are emitted and .beta. depending on the work function at the tip end of the emitter and the field intensity generated by the configuration of the emitter near its tip end.
When the gate voltage shown in FIG. 2a is applied to the gate electrode, the emission current increases as the gate voltage increases, as shown in FIG. 2b. Thereafter, the emission current gradually decreases toward a certain value and becomes stable. The change in the emission current is caused because a small amount of gases remaining in the vacuum is adsorbed to the tip end of the emitter thereby to reduce the effective area for emitting electrons or increase the work function of the surface of the emitter, changing the constants .alpha., .beta. of the Fowler-Nordheim equation.
One way of stabilizing the varying emission current is to provide transistors and resistor layers and use a constant-current regulated power supply circuit. For example, Japanese laid-open patent publication No. 1994-290701 discloses resistors disposed between emitter electrodes and emitters for stabilizing the varying emission current.
The above process of stabilizing the varying emission current by providing transistors and resistor layers and using a constant-current regulated power supply circuit is problematic in that a method of driving the device is complex, it is difficult to operate the device at high frequencies because of the capacitance and resistance of the added transistors, and the consumption by the device of electric energy is large.
Japanese laid-open patent publication No. 1992-332423 discloses a gate electrode made of a material which can be combined with oxygen more strongly than the material of the emitter, for thereby preventing active residual gases such as of oxygen from being adsorbed to the emitter surface and reacting with the emitter to increase the work function.
However, if the gate electrode is made of a material which can be combined with oxygen more strongly than the material of the emitter in order to stabilize the varying emission current, there is a limited choice of materials available for the gate electrode, which can be combined with oxygen more strongly than the material of the emitter and lend themselves to microfabrication, and the gate electrode tends to become lower in reliability when oxidized.